In the recent drive for higher integration and operating speeds in LSI devices, the pattern rule is made drastically finer. The photolithography succeeded in forming finer feature size patterns by reducing the wavelength of exposure light. Efforts are now made to jump from the ArF excimer laser of wavelength 193 nm to extreme ultraviolet (EUV) of wavelength 13.5 nm. The light source for EUV is of a considerably low power while the development works of appropriate multilayer reflection mirrors, multilayer reflection masks, and photoresists are retarded. With the advent of the immersion lithography, the projection lens which is one key for implementing miniaturization has a numerical aperture (NA) in excess of 1.0 and has already reached the maximum NA which is governed by the reflective index of water. With the prior art methods, no further miniaturization is possible.
The process that now draws attention under the above-discussed circumstances is a double patterning process involving a first set of exposure and development to form a first pattern and a second set of exposure and development to form a pattern between features of the first pattern. See Proc. SPIE Vol. 5992, 59921Q-1-16 (2005). A number of double patterning processes are proposed. One exemplary process involves a first set of exposure and development to form a photoresist pattern having lines and spaces at intervals of 1:3, processing the underlying layer of hard mask by dry etching, applying another layer of hard mask thereon, a second set of exposure and development of a photoresist film to form a line pattern in the spaces of the first exposure, and processing the hard mask by dry etching, thereby forming a line-and-space pattern at a half pitch of the first pattern. An alternative process involves a first set of exposure and development to form a photoresist pattern having spaces and lines at intervals of 1:3, processing the underlying layer of hard mask by dry etching, applying a photoresist layer thereon, a second set of exposure and development to form a second space pattern on the remaining hard mask portion, and processing the hard mask by dry etching. In either process, the hard mask is processed by two dry etchings.
While the former process requires two applications of hard mask, the latter process uses only one layer of hard mask, but requires to form a trench pattern which is difficult to resolve as compared with the line pattern. The latter process includes the use of a negative resist material in forming the trench pattern. This allows for use of high contrast light as in the formation of lines as a positive pattern. However, since the negative resist material has a lower dissolution contrast than the positive resist material, a comparison of the formation of lines from the positive resist material with the formation of a trench pattern of the same size from the negative resist material reveals that the resolution achieved with the negative resist material is lower. After a wide trench pattern is formed from the positive resist material by the latter process, there may be applied a thermal flow method of heating the substrate for shrinkage of the trench pattern, or a RELACS® method of coating a water-soluble film on the trench pattern as developed and heating to induce crosslinking at the resist film surface for achieving shrinkage of the trench pattern. These have the drawbacks that the proximity bias is degraded and the process is further complicated, leading to reduced throughputs.
Both the former and latter processes require two etchings for substrate processing, leaving the issues of a reduced throughput and deformation and misregistration of the pattern by two etchings.
One method that proceeds with a single etching is by using a negative resist material in a first exposure and a positive resist material in a second exposure. Another method is by using a positive resist material in a first exposure and a negative resist material in an alcohol that does not dissolve away the positive resist material in a second exposure. Since negative resist materials with low resolution are used, these methods entail degradation of resolution.
Now under investigation is the resist pattern freezing technology involving forming a first resist pattern on a substrate, taking any suitable means for insolubilizing the resist pattern with respect to the resist solvent and alkaline developer, applying a second resist thereon, and forming a second resist pattern in space portions of the first resist pattern. With this freezing technology, etching of the substrate is required only once, leading to improved throughputs and avoiding the problem of misregistration due to stress relaxation of the hard mask during etching.
With respect to the freezing technology, a number of techniques are reported including thermal insolubilization, coating of a cover film and thermal insolubilization, insolubilization by irradiation of light having an extremely short wavelength, for example, of 172 nm, insolubilization by ion implantation, insolubilization through formation of thin-film oxide by CVD, and insolubilization by light irradiation and special gas treatment. These insolubilization methods, which involve heat treatment at elevated temperature, give rise to problems of pattern deformation, especially film slimming, and size narrowing or widening, which must be overcome.
The critical issue associated with double patterning is an overlay accuracy between first and second patterns. Since the magnitude of misregistration is reflected by a variation of line size, an attempt to form 32-nm lines at an accuracy of 10%, for example, requires an overlay accuracy within 3.2 nm. Since currently available scanners have an overlay accuracy of the order of 8 nm, a significant improvement in accuracy is necessary.
If first exposure is followed by second exposure at a half-pitch shifted position, the optical energy of second exposure offsets the optical energy of first exposure so that the contrast becomes zero. If a contrast enhancement layer (CEL) is formed on the resist film, the incident light to the resist film becomes nonlinear so that the first and second exposures do not offset each other. Thus an image having a half pitch is formed. See Jpn. J. Appl. Phy. Vol. 33 (1994) p 6874-6877. It is expected that similar effects are produced by using an acid generator capable of two photon absorption to provide a nonlinear contrast. Using this double imaging method, the resolution can be doubled through two exposure steps and a single development.
A resist material having both positive and negative properties has been proposed. This resist material displays the positive tone response that it is substantially insoluble in alkaline developer where it receives a low exposure dose, but increases its alkaline dissolution rate as the exposure dose is increased, and the negative tone response that it starts reducing its alkaline dissolution rate as the exposure dose is further increased. Lithographic processing of such a positive/negative resist material can produce a resolution twice that of the mask pattern since those portions of resist film having received low and high exposure doses are left after development. Known positive/negative resist materials include a positive/negative hybrid resist composition obtained by adding a crosslinker to a positive resist material as described in U.S. Pat. No. 6,114,082 and Proc. SPIE Vol. 3678, p 348 (1999), and a positive/negative hybrid resist composition utilizing competitive positive and negative reactions in the co-presence of a benzyl alcohol and an acetal acid labile group as described in JP-A 2003-005353. The positive/negative hybrid resist composition permits a pattern to be formed at a double resolution by an ordinary process involving single exposure and single development.
The cost of double patterning is a problem. While the complexity of additional steps during double patterning is considered problematic, double exposures constitute the majority of cost. If a double pattern can be formed through a single exposure, then the process becomes of the lowest cost. The double patterning or double imaging process cannot be established unless the alignment accuracy of the exposure tool is significantly improved. On the other hand, if the positive/negative hybrid resist material is applicable to the ArF immersion lithography, the resolution can be doubled through a single exposure, overcoming the problems of cost and alignment accuracy of the exposure tool. Then the technology becomes a promising candidate for 32 nm and 22 nm.
Photobase generators proposed heretofore are nitrobenzyl carbamates (J. Am. Chem. Soc. 1991, 113, p 4303-4313). Photoresist materials having the photobase generator added thereto are described in U.S. Pat. No. 5,545,509 and Proc. SPIE Vol. 1466 p 75 (1991). Also a resist composition is obtained by adding a photobase generator to an ordinary positive photoresist material comprising a base polymer having acid labile groups and a photoacid generator (JP-A H10-083079).